Aluminum alloy films for use in display devices are mainly used as electrodes and wiring materials. Examples of the electrodes and wiring materials include gate, source, and drain electrodes for a thin film transistor and a wiring material in a liquid crystal display (LDC); gate, source, and drain electrodes for a thin film transistor and a wiring material in an organic EL (GELD); cathode and gate electrodes and a wiring material in a field emission display (FED); an anode electrode and a wiring material in a vacuum fluorescent display (VFD); an address electrode and a wiring material in a plasma display panel (PDP); and a back electrode in an inorganic EL.
Aluminum alloy films for use in semiconductor device are mainly used as electrodes and wiring materials. Examples of the electrodes and wiring materials include emitter and collector electrodes and a wiring material for an insulated gate bipolar transistor (IGBT).
Hereinafter, while a liquid crystal display is representatively described as a liquid crystal display device, the present invention is not limited thereto.
Large-sized liquid crystal displays are widely used as main display devices because of advancement in low power consumption technology. A liquid crystal display having a size of more than 100 inches are now commercialized. There are various types of liquid crystal displays having different operating principles. Among them, active-matrix liquid crystal displays including thin film transistors (hereinafter, referred to as “TFTs”) used for the switching of pixels are most-widely used because they have high-precision image qualities and can display fast moving images. In liquid crystal displays required to have lower power consumption and higher switching speeds of pixels, TFTs including semiconductor layers composed of polycrystalline silicon and continuous grain silicon are used.
For example, active-matrix liquid crystal displays include TFT substrates including TFTs serving as switching elements, pixel electrodes formed of conductive oxide films, and wiring, such as scan lines and signal lines. Scan lines and signal lines are electrically connected to pixel electrodes. Wiring materials constituting scan lines and signal lines are formed of Al-based alloy thin films.
The structure of a core portion of a TFT substrate including a semiconductor layer composed of hydrogenated amorphous silicon is described below with reference to FIG. 1.
As illustrated in FIG. 1, a scan line 25 is arranged on a glass substrate 21a. Part of the scan line 25 functions as a gate electrode 26 that controls the on/off state of a TFT. The gate electrode 26 is electrically insulated with a gate insulating film 27 (e.g., silicon nitride film). A semiconductor silicon layer 30 is arranged as a channel layer on the gate insulating film 27. Furthermore, a passivation film 31 (e.g., silicon nitride film) and so forth are arranged. The semiconductor silicon layer 30 is connected to a source electrode 28 and a drain electrode 29 with a low-resistance silicon layer 32 and has electrical conductivity.
The drain electrode 29 has a structure (called a “direct contact (DC)”) in which the drain electrode is in direct contact with a transparent electrode 35 composed of indium tin oxide (ITO). As an electrode wiring material used for the direct contact, Al alloys described in patent literatures 1 to 5 are exemplified because Al has low electrical resistivity and excellent micro-fabrication property. These Al alloys are each directly connected to a transparent conductive oxide film constituting a transparent electrode or a semiconductor silicon layer without using a barrier metal layer composed of a refractory metal, such as Mo, Cr, Ti, and W.
These wiring films and electrodes 25 to 32 are covered with an insulating passivation film 33 composed of, for example, silicon nitride and supply electricity to the drain electrode 29 through the transparent electrode 35.
To ensure stable operating characteristics of the TFT illustrated in FIG. 1, in particular, it is necessary to increase the mobility of carriers (electrons and holes) in the semiconductor silicon layer 30. Thus, a production process of a liquid crystal display or the like includes a heat treatment step of heat treating a TFT, thereby resulting in microcrystallization or polycrystallization of the whole or part of the semiconductor silicon layer 30 having an amorphous structure. This increases the carrier mobility, improving the response speed of the TFT.
In the fabrication process of the TFT, for example, the deposition of the insulating passivation film 33 is performed at a relatively low temperature of about 250° C. to 350° C. To improve the stability of a TFT substrate (a liquid crystal display-driving portion in which TFTs are arranged in an array), high-temperature heat treatment at about 450° C. or higher is conducted in some cases. In the actual production of a TFT, a TFT substrate, and a liquid crystal display, such low- or high-temperature heat treatment is conducted twice or more in some cases.
However, in the case where the heat treatment temperature in a production process is a high temperature, such as about 450° C. or higher, and where such high-temperature heat treatment is conducted for extended periods of time, the heat treatment causes delamination of the thin layers illustrated in FIG. 1 and atomic interdiffusion between thin layers in contact with each other, thereby deteriorating the thin layers. Heat treatment has thus been conducted only at a temperature of at most 300° C. or lower. If anything, the fact is that research and development on wiring materials used for TFTs and structures of display devices has been intensively conducted even if the heat treatment temperature is minimized. This is because the entire production process of a TFT is considered to be ideally performed at room temperature from the technical point of view.
For example, in patent literature 1 described above, the entire or part of a dissolved element in an Al alloy film are precipitated in the form of metal compound by heat treatment conducted at 100° C. to 600° C. to obtain an Al alloy film having electrical resistivity of 10 μΩcm or less. However, experimental data disclosed in its embodiment are those heat-treated at 500° C. at the highest. Patent literature 1 fails to disclose heat resistance of an Al alloy film treated at temperatures higher than 500° C. Similarly, in an example of patent literature 2, results when an Al alloy film is heat-treated up to 500° C. are described, whereas no result when heat treatment is performed at a higher temperature is described. Patent literatures 3-5 describes evaluation results of heat resistance at 450° C. or lower. However, heat resistance when heat treatment is performed at a higher temperature is not evaluated at all. Needless to say, heat resistance when the film is repeatedly exposed to such a high temperature is not considered at all.
In the meantime, patent literature 6 discloses an Al alloy film having superior dry-etching fabrication property. However, results when an Al alloy film is heat-treated at 350° C., whereas no result when heat treatment is performed at 450° C. or higher temperatures is described.